Electrophotography, because of its simplicity, has during the last decade found rapid acceptance in the office copying and reproduction fields. The first electrophotographic process in its basic form is described by Chester F. Carlson in U.S. Pat. No. 2,297,691. Since Carlson's discovery, many other reproduction processes based on the use of photoconductive materials have followed. Such methods have in common the use of toners, and if the photoconductive surface is to be reusable, it must be kept clean of toner accumulation. The present invention solves this problem.
In the type of electrophotography known as xerography, the essential element is the reusable electrophotoconductive insulating layer. A latent electrostatic image is formed by first charging and then exposing the electrophotoconductive insulating surface to a light pattern. The image is then developed with a thermoplastic electroscopic material, known as toner, and this material is transferred to and then fixed to paper.
The methods employed to develop images in electrophotographic printing processes are many and varied. They include cascade development as described in U.S. Pat. No. 2,618,552, powder cloud development as described in U.S. Pat. No. 2,221,776, magnetic brush development as described in U.S. Pat. No. 2,874,063 and other methods including fur brush development, donor belt development, impression development and liquid spray development. The two methods most frequently employed in commercial office copying machines which make use of reusable electrophotographic insulators are the cascade mode of development and magnetic brush development. The toner particles applied in these development processes consist of one or more thermoplastic resin binder materials. The most frequently employed thermoplastic resin binder is a copolymer of styrene and butyl methacrylate, of the type shown, for instance, in U.S. Pat. No. 2,940,934. Other thermoplastic resin binders are also used, either alone or in mixtures, for example, polystyrene, polymethyl styrene, polylethyl methacrylate, polybutyl methacrylate, polyvinyl butyral, coumarone-indene resins, copolymers of n- and p-vinyltoluene and butadiene, thermoplastic polyamide resin, epoxy resins, rosin, rosin esters, and the like. The binder is mixed with about 8% of a coloring pigment, for example, carbon black, iron or a dye such as nigrosine, so that a colored image can be easily heat fused onto a copy sheet.
The transfer of the toner to the paper is by electrical attraction. Electrical transfer is accomplished by placing the paper in contact with the imaged area of the plate, charging the paper electrically with the same polarity as that of the latent image, and then stripping the paper from the plate. The charge applied to the paper overcomes the attraction of the latent image for the toner particles and pulls them onto the paper. Another technique for electrostatic transfer utilize a semiconductive roll. A dc potential of the correct sign and voltage is applied between the roll and the electrode of the reusable electrophotoconductive insulating layer.
Complete transfer of toner from the surface of the reusable photoconductive insulating layer to the paper is not accomplished by these transfer methods. Accordingly, a fraction of the toner remains behind on the surface of the reusable electrophotoconductive insulating layer, and this residual toner must be removed prior to the next cycle. In automatic machines cleaning of the residual toner is usually accomplished by a rotating fur brush combined with a vacuum suction. The fur can be natural such as rabbits fur, or synthetic such as nylon, or Dynel, which is a copolymer of vinyl chloride and acrylonitrile. Another method for cleaning residual toner from the surface of the reusable electrophotoconductive layer is web cleaning.
These methods of cleaning are efficient in that they remove all but a small fraction of the residual toner. The frictional forces generated during the cleaning, however, promote the wetting of the surface of the photoconductive insulating layer by the thermoplastic resins of this small fraction of toner and this in turn results in a formation of a film of toner on the surface of the reusable layer. During recycling toner-filming continues and eventually a layer of pigment and thermoplastic resin of such dimensions is present on the surface of the reusable element that copy quality is seriously impaired. This deleterious effect occurs after several hundred or several thousand cycles.
After copy quality is impaired, it becomes necessary to clean or replace the electrophotoconductive insulating surface. This is inconvenient and expensive in time and material, and the solution of this problem is an object of the present invention.